This invention concerns a two-stage flotation purification system for removing suspended NaCl and Na.sub.2 SO.sub.4 impurities from aqueous NaOH having a concentration of less than 52 percent by weight of NaOH. The process is well suited to purification of caustic soda manufactured by electrolysis of brine in cells utilizing diaphragm partitions.
It is known to remove suspended NaCl and Na.sub.2 SO.sub.4 impurities from aqueous sodium hydroxide by filtration and by centrifugation. Filtration purification is a costly, difficult to operate procedure. Centrifugation is difficult to control because it is so sensitive to crystal size.
Purification of aqueous sodium hydroxide by flotation of NaCl has been disclosed in Japanese patent application No. 52-69895. However, that purification relies on use of organic surface active agents and does not contemplate the two-stage, temperature-related separation described herein. Use of organic surface active agents is undesirable because of the difficulty of removing them from recycled NaCl and from NaOH product. The surface active agents are impurities in the NaOH product. Also, they may damage cell anodes when returned with recycled NaCl to the electrolysis cells.
U.S. Pat. No. 4,065,270 discloses the flotation separation of crystals of impurities from a slurry of sodium hydroxide hydrate crystals. The patent does not contemplate separation of impurity crystals from aqueous sodium hydroxide solution nor does it contemplate the two-stage, temperature-related separation described herein. Production of sodium hydroxide hydrate crystals as taught by the patent is avoided in the process of this invention wherein such crystals would become entrained with those of the very impurities from which NaOH is desired to be separated.